A key requirement for the packaging of a microelectronic device is that signals move through the package's conductive interconnects such that the electrical interconnection causes minimal change in the signals. It is difficult, however, to fabricate microelectronic packages to achieve minimal signal change at higher frequencies, i.e., frequencies in the Gigahertz (GHz) range. Along with limited frequency ranges, conventional microelectronic packages have excessive transmission and reflective losses, limited input/output isolation, high cost, and limited reliability, resulting in a lack of general applicability.
It is even more difficult to fabricate high frequency microelectronic packages which can be connected via conductive leads to a next level of assembly such as a circuit board or other microelectronics package. Conventional leaded microelectronic packages, including those having stripline transmission lines, experience unacceptable insertion and return signal losses which change the signals at high frequencies. The unacceptable electrical properties are due to impedance discontinuities caused by the leads, and interconnections between the leads and mating lead pads on the package and on the next level of assembly.
The above-listed related applications and patents disclose improved microelectronic packages that address one or more of the problems due to limitations and disadvantages of the related art. For example, U.S. Pat. No. 5,448,826 shows a ceramic microelectronics package 100 suitable for housing high-frequency electronic devices, as shown in FIGS. 1-3 herein. Package 100 includes a base 102, first attaching means 104, a ceramic radio-frequency (RF) circuit substrate 106, second attaching means 108, a ceramic seal ring substrate 110, non-conducting third attaching means 112, and a ceramic lid 114. Package 100 is used as an electronic interconnect housing for a high-frequency electronic device or component 116 mounted to base 102. Device 116 is received within a cavity 120 formed within circuit substrate 106. A plurality of conductive traces 122 patterned on circuit substrate 106 provide electrical connections between device 116 and an external device (not shown). Seal ring substrate 110 has a cavity 124 larger than cavity 120. Device 116 is an exemplary high-frequency electronic device housed within package 100, and it is understood that device 116 represents any high-frequency electronic device or component. Package 100, and the process for making package 100, are fully disclosed in the '826 patent, which is incorporated herein by reference.
Each conductive trace 122 in package 100 forms a portion of a microstrip transmission line. "Microstrip transmission line" is defined herein as being a conductor suspended above a ground plane and separated from the ground plane by a dielectric. Each conductive trace 122 is a conductor, base 102 forms a ground plane, ceramic circuit substrate 106 is a dielectric, and each trace 122 is suspended above base 102 and is separated from base 102 by substrate 106. Thus, each conductive trace 122 forms a portion of a microstrip transmission line which propagates a signal between the external device and device 116 as electric and magnetic fields. The impedance of microstrip transmission lines is a function of the dielectric value of substrate 106, the width of traces 122, the gap to the top surface of the ground plane formed by base 102, and the thickness of substrate 106 below traces 122. Mathematical formula are known which approximate the impedance for given dielectric and conductor parameters and geometries.
Each microstrip transmission line in package 100 has the form of a microstrip, embedded microstrip, microstrip transmission line as the line transitions from outside package 100, beneath seal ring substrate 110, to inside package 100. An "embedded microstrip transmission line" is a microstrip transmission line located beneath a second dielectric material, i.e., sandwiched between two dielectric layers or materials. Since ceramic circuit substrate 106 and seal ring substrate 110 are both formed from dielectric material, the middle portion of each microstrip transmission line passing beneath seal ring substrate 110 has the form of an embedded microstrip transmission line, and the portions of each microstrip transmission line on either side of substrate 110 (i.e., not beneath substrate 110) have the form of regular (i.e., non-embedded) microstrip transmission lines.
The microstrip transmission lines in package 100, including conductive traces 122, transition through microstrip feed-throughs when entering and exiting the package. A "feed-through" is an area within a dielectric through which a portion of a conductive trace which passes, such as from the interior of a package to the exterior of a package. For example, the presence of seal ring substrate 110 causes the microstrip transmission lines to pass through such an area as the transmission lines transition from outside package 100 to inside package 100. Thus, the feed-throughs of package 100 can be referred to as microstrip feed-throughs.
Package 100 can be referred to as a high-frequency broadband microelectronics package. "Broadband" refers to the ability of broadband signals (DC to GHz frequencies) to move through the package's conductive interconnects such that the electrical connections cause minimal change in the signals. Each trace 122 includes an outer conductive bonding pad for electrical connection to the next level of assembly (e.g., a circuit board or another package) by wire or ribbon bonding the outer bonding pad to a corresponding or mating pad on the next level of assembly. For example, the mating pads may be joined by a thin gold wire tack-welded to each pad and passing over a gap between the pads. The only deviations from a continuous transmission line (which would give an optimal frequency response) is the error in matching the width of the bond wire to the width of the transmission line conductor, the weld joints, the gap in the dielectric, and a small gap in the ground plane. The use of wire or ribbon bonds to perform these interconnections provides the transmission lines with an impedance compatible with high frequencies.
Package 100, however, is not designed to be connected to the next level of assembly using conductive leads in such high-frequency applications. A "lead" is typically a generally rectangular piece of metal conductor which can be electrically connected by braising or soldering the lead between a lead pad formed on a microelectronics package and a mating lead pad formed on a next-level circuit board or another package. A "lead pad" is an anchor pad formed in the transmission line for receiving the lead. For example, on a package including a 10 mil (i.e., 0.010") thick, 96% alumina ceramic circuit substrate, the calculated width of the conductor for a microstrip transmission line would be 10 mil. The ideal lead would also be 10 mil wide, and would be attached to the conductor without creating a lump.
In practice, however, the end of the conductor where the transmission line approaches the edge of the package is widened to form a lead or anchor pad 25 mil wide to provide a strong mechanical bond between the lead and the lead pad. The lead pad is made wider than the lead to allow for some mis-alignment during assembly and for an attachment fillet. The lead also has a finite thickness creating a change in thickness of the transmission line. The braise or solder used to connect the lead between the lead pads forms lumps. The change in width of the transmission line at the anchor pad, the thickness change of the lead, and the braise or solder lumps have combined to limit the upper frequency of leaded microelectronics packages to lower Gigahertz frequencies. For frequencies in the mid Gigahertz range and above, the lead and interconnections between the lead and the lead pads, create impedance discontinuities having unacceptable electrical properties which cause the signals to experience more than minimal changes. Thus, microelectronics packages, such as package 100 operating in the higher frequencies, e.g., above 20-23 GHz, have used wire or ribbon bonding to interconnect the package conductors with mating conductors on the next level of assembly.
The use of wire bonding, however, to electrically interconnect the outer pads of traces 122 to the mating pads on the next level of assembly has certain disadvantages in comparison to connections made using conductive leads. First, wire bonding is a relatively labor intensive operation when performed manually, and requires relatively expensive automatic wire bonding equipment when performed automatically. Second, wire bonding requires relatively expensive materials. Third, wire bonding requires relatively tight dimensional tolerances between the corresponding pads which impose restraints on the manufacturing processes. These disadvantages combine to increase the costs associated with using package 100 in high-frequency applications. The increased costs are especially disadvantageous in high-volume, low-cost applications, such as in high-volume, low-cost consumer electronics applications.
Thus, it would be desirable to provide a high-frequency microelectronics package suitable for housing a high-frequency electronic device operating at high frequencies, e.g., in and above the Gigahertz range, which can be connected to the next level of assembly using conductive leads. Such a package would reduce labor and material requirements, eliminate the need for automatic wire bonding equipment, and loosen the dimensional tolerances required of the assembly process compared to packages designed to be wire or ribbon bonded to the next level of assembly.